Recording materials utilizing a combination of an electron-donating colorless dye and an electron-accepting compound have already been well-known as pressure sensitive paper, heat sensitive paper, light- and pressure-sensitive paper, electro thermo-recording paper and so on.
Detailed descriptions of these papers are given, for instance, in British Pat. No. 2,140,449, U.S. Pat. Nos. 4,480,092 and 4,436,920, Japanese Patent Publication No. 23922/85, Japanese Patent Application (OPI) Nos. 179836/82, 123556/85 and 123557/85 (the term "OPI" as used herein means an "published unexamined Japanese patent application"), and so on.
The recording materials must have properties of (1) producing a sufficiently high color density of the developed image at a sufficiently high speed, (2) forming no fog, (3) producing a developed image which retains sufficient fastness after color development, (4) producing a developed image of an appropriate hue, and showing an aptitude for copying machines, (5) having a high signal to noise (S/N) ratio, (6) producing a developed color image having a sufficiently high chemical resistance, (7) being produced from dyes which are readily able to be dissolved in an organic solvent, and so on. However, recording materials which satisfy all of these requirements to perfection have not been obtained yet.
In particular, studies on improvements in characteristics of such recording materials have been actively pursued in response to the speeding-up of the recording system and the diversification of requirements in recent years.
As for the compounds capable of developing a blue to bluish violet color, diphenylmethane type compounds, triphenylmethane type compounds, phthalide compounds, Lueco-methylene Blue type compounds and the like have so far been known. However, these compounds possess their individual defects.
For instance, the indolylphthalide compounds as described in U.S. Pat. Nos. 3,829,322 and 4,062,866 develop a color image having a satisfactory hue and satisfactory light fastness at a high rate of development, but the image produced is insufficient in stability. In particular, when preserved in cases or files made of transparent polyvinyl chloride commonly employed for storage of records, the image is seriously inferior in light-fastness and also undergoes smearing due to plasticizers.
Our attention has been directed to various characteristics of electron-donating colorless dyes and electron-accepting compounds, including solubilities in oil, solubilities in water, partition coefficients, pKa values, polarities of substituent groups, positions of substituent groups, changes in crystallinity and solubility upon use in a warmed condition and so on, and with which the development of excellent substances for recording materials and recording materials has been sought.